And then there's a new requirement of iterator stability for that case?
Sounds like a tricky sharp edge...

On Sun, 14 Sept 2025, 18:30 Peter Neiss via Std-Proposals, <
std-proposals_at_[hidden]> wrote:

> Hello Phil,
> i disagree with the inplace_set_intersection solution. I read up on weak
> ordering. If the input ranges overlap, the intersection is part of this
> overlap which either does not change or is already read before overwritten
> in a good algorithm.
> I think we can just improve the std::set_intersection preconditions and
> stuff that already works also becomes legal and is not arbitrarily illegal
> in the standard.
> Regards,
> Peter
>
> Am So., 14. Sept. 2025 um 19:22 Uhr schrieb Phil Endecott via
> Std-Proposals <std-proposals_at_[hidden]>:
>
>> Peter Neiss wrote:
>> > The std::set_intersection algorithm forbids overlap of the input ranges
>> > with the output range, but if first1 or first2 and result would be
>> equal,
>> > the algorithm could still work
>>
>> I suggest a new name and signature, rather than changing
>> std::set_intersection:
>>
>> template <class It1, class It2>
>> It1 inplace_set_intersection(It1 first1, It1 last1, It2 first2, It2
>> last2);
>>
>> Range first2...last2 is read from sequentially and not modified;
>> I guess It2 is an InputItrator.
>>
>> Range first1...last1 is read from and modified; the result is the range
>> from first1 to the returned iterator. Maybe It1 needs to be a
>> ForwardIterator? Not sure.
>>
>> There are a couple of choices for how the range between the end
>> of the result and last1 is left; the elements can either be moved
>> or swapped. I guess this range should be left valid-but-undefined.
>>
>> I'm not sure if this is worth having in the standard library; anyone
>> could write it themselves in a few minutes. Maybe its value would
>> be to prevent people from wrongly assuming that std::set_intersection
>> can already be used in this way!
>>
>>
>> Regards, Phil.
>>
>>
>>
>>
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